Many industries, including the assembly processing, grocery and food processing industries, utilize an identification system in which the products are marked with a bar code symbol consisting of a series of lines and spaces of varying widths. A number of different bar code readers and laser scanning systems have been developed to decode the symbol pattern to a multiple digit representation for inventory, production tracking, and for check out or sales purposes. Optical scanners are available in a variety of configurations, some of which are built into a fixed scanning station and others of which are portable. The portability of an optical scanning head provides a number of advantages, including the ability to inventory products on shelves and to track portable items such as files or small equipment. A number of these portable scanning heads incorporate laser diodes which permit the user to scan the bar code symbols at variable distances from the surface on which the bar code is imprinted. A disadvantage of laser scanners is that they are expensive to manufacture.
Another type of bar code scanner which can be incorporated into a portable system uses light emitting diodes (LED) as a light source and charge couple devices (CCD) as detectors. This class of bar code scanners is generally known as "CCD scanners". While conventional CCD scanners have the advantage of being less expensive to manufacture, they limit the user to scanning the bar code by either contacting the surface on which the bar code is imprinted or maintaining a distance of no more than one and one-half inches away from the bar code, which creates a further limitation in that it cannot read a bar code any longer than the window or housing width of the scanning head. Thus, the CCD scanner does not provide the comfort or versatility of the laser scanner which permits variable distance scanning of bar code symbols which may be wider than the window or housing width.
As the use of bar codes for information storage becomes more prevalent, a goal of the optical scanning technology will be to obtain the highest possible bar density in the area that can be scanned in a single "snapshot". Improvements have been disclosed in the present inventor's issued U.S. Pat. Nos. 5,291,009, 5,349,172, and 5,354,977 and co-pending applications, Ser. Nos. 07/843,266, 08/058,951, and 08/059,322, which provide increased depth of field in CCD scanners to allow scanning of larger area symbols. This, in turn, allows more information to be acquired in a single snap shot. However, in order to further optimize the capability of decoding these symbols, it is necessary to ensure that the increased density of the code within a single snap shot does not reduce the scanner's reliability in decoding it.